Phenol is an important product in the chemical industry and is useful in, for example, the production of phenolic resins, bisphenol A, ε-caprolactam, adipic acid, and plasticizers.
Currently, the most common route for the production of phenol is the Hock process. This is a three-step process in which the first step involves alkylation of benzene with propylene to produce cumene, followed by oxidation of the cumene to the corresponding hydroperoxide and then cleavage of the hydroperoxide to produce equimolar amounts of phenol and acetone. However, the world demand for phenol is growing more rapidly than that for acetone. In addition, the cost of propylene is likely to increase, due to a developing shortage of propylene. Thus, a process that uses higher alkenes instead of propylene as feed and coproduces higher ketones, rather than acetone, may be an attractive alternative route to the production of phenols.
For example, oxidation of the cyclohexylbenzene (analogous to cumene oxidation) could offer an alternative route for phenol production without the problem of acetone co-production. This alternative route proceeds through cyclohexylbenzene hydroperoxide, which is cleaved to produce phenol and cyclohexanone in substantially equimolar amounts.
However, one problem in producing phenol by way of the cleavage of cyclohexylbenzene hydroperoxide is that the cyclohexanone and phenol produce an azeotropic mixture composed of 28 wt % cyclohexanone and 72 wt % phenol. Thus, one cannot recover all of the cyclohexanone, or any of the phenol, from the cleavage effluent as a saleable product by simple distillation. Moreover, although cyclohexanone is a valuable product with a growing market, there is currently no large worldwide merchant market for cyclohexanone; most cyclohexanone is made as an intermediate and consumed on the spot. In some cases, therefore, it may be desirable to increase the amount of phenol in the product mix from the cleavage of cyclohexylbenzene hydroperoxide or even produce all phenol with no cyclohexanone.
For example, U.S. Published Patent Application No. 2011/0105805 discloses a process for producing phenol by oxidation of cyclohexylbenzene to cyclohexylbenzene hydroperoxide followed by cleavage of the cyclohexylbenzene hydroperoxide, in which some or all of the effluent from the cleavage step is subjected to a selective dehydrogenation step to convert at least a portion of the cyclohexanone in the effluent portion into phenol and hydrogen. Where only part of the cleavage effluent is dehydrogenated, the effluent is initially subjected to at least a first separation step to recover some or all of the phenol from the effluent, typically so that the effluent stream fed to said dehydrogenation reaction contains less than 50 wt %, for example less than 30 wt %, such as less than 1 wt %, phenol. Additional distillation steps can be used to remove components boiling below 155° C. (as measured at 101 kPa), such as benzene and cyclohexene, and/or components boiling above 185° C. (as measured at 101 kPa), such as 2-phenyl phenol and diphenyl ether, prior to feeding the effluent stream to the dehydrogenation reaction.
One problem with the process proposed in U.S. Published Patent Application No. 2011/0105805 is that expensive vacuum and/or extractive distillation methods are required to recover the phenol before the remainder of the effluent is fed to the dehydrogenation reaction. To address this problem, the present invention proposes alternative approach to increasing the amount of phenol in the product mix from the cleavage of cyclohexylbenzene hydroperoxide. Thus, in accordance with the invention, cyclohexanone is separated from the cleavage effluent, typically by conventional distillation and preferably to reduce the cyclohexanone content of the effluent to approach the azeotropic amount of 28 wt %. According to market conditions, the cyclohexanone can then be recovered as a usable product or dehydrogenated to produce phenol, whereas the remainder of the cleavage effluent, potentially an azeotropic mixture of phenol and cyclohexanone, is subjected to selective dehydrogenation to convert the cyclohexanone to phenol.
Cyclohexanone is typically produced by the oxidation of cyclohexane, or the hydrogenation of phenol. These methods can also generate various contaminants that are difficult to separate from the desired product, and that can render the cyclohexanone product substandard or unusable to downstream processes, for example in the manufacture of caprolactam or adipic acid, or further using those derivatives in the production of one or another type of nylon. Such contaminants include butylcyclohexylether, pentylcyclohexane, cyclohexyl acetate, pentanal, valeric acid and butyric acid.
The production of cyclohexanone from cyclohexylbenzene (with phenol as a co-product) is an emerging technology. The production of cyclohexanone from cyclohexylbenzene also produces various contaminants that are difficult to separate from the desired products. However, the nature of those contaminants and the separations involved are significantly different than those involved in the conventional production of cyclohexanone from cyclohexane or phenol. For example, hydroalkylation of benzene produces significant amounts of, inter alia, cyclohexane and lesser amounts of methylcyclopentane, cyclohexene, phenylcyclohexene, and phenylcyclohexyldiene. Similarly, the oxidation of cyclohexylbenzene typically produces peroxide species alien to conventional processes for making cyclohexanone, such as the desired cyclohexyl-1-phenyl-1-hydroperoxide (CHBHP), and undesired byproduct hydroperoxides such as cyclohexyl-1-phenyl-2-hydroperoxide, cyclohexyl-1-phenyl-3-hydroperoxide and cyclohexyl-1-phenyl-4-hydroperoxide. Finally, the cleavage of these various hydroperoxides produces, as both the product of the undesired hydroperoxides and the undesired byproducts of the desired CHBHP, a wide variety of contaminant species are not produced by the chemistry and technology of the cyclohexane oxidation or phenol hydrogenation processes, such as cyclohexanedione and hydroxycyclohexanone.